Recycling of waste materials is highly desirable from many viewpoints, not the least of which are financial and ecological. Properly sorted recyclable materials can often be sold for significant revenue. Many of the more valuable recyclable materials do not biodegrade within a short period, and so their recycling significantly reduces the strain on local landfills and ultimately the environment.
Typically, waste streams are composed of a variety of types of waste materials. One such waste stream is generated from the recovery and recycling of automobiles or other large machinery and appliances. Other waste streams may include electronic components, building components, or other industrial waste streams. These materials are generally of value only when they have been separated into like-type materials. However, in many instances, no cost-effective methods are available to effectively sort waste streams that contain diverse materials. This deficiency has been particularly true for non-ferrous materials, and particularly for non-metallic materials, such as high density plastics, and non-ferrous metals, including copper wiring. For example, one approach to recycling plastics has been to station a number of laborers along a sorting line, each of whom manually sorts through shredded waste and manually selects the desired recyclables from the sorting line. This approach is not sustainable in most economics since the labor cost component is too high. Also, while ferrous recycling has been automated for some time, mainly through the use of magnets, this technique plainly is ineffective for sorting non-ferrous materials. Again, labor-intensive manual processing has been employed to recover wiring and other non-ferrous metal materials. Because of the cost of labor, many of these manual processes are conducted in other countries and transporting the materials to and from these countries adds to the cost.
A variety of plastics may be contained within a waste stream. Some such plastics include polypropylene (PP); polyethylene (PE); acrylonitrile butadiene styrene (ABS); polystyrene (PS), including high impact polystyrene (HIPS), and polyvinyl chloride (PVC). Other materials, such as wood, rubber, and foam may be present. Typically, these materials are less valuable, and ultimately make up the waste materials from the recovery process. Of course, in some cases, these materials may be recovered as useful depending on the application.
Many processes for identifying and separating materials are know in the art. However, not all processes are efficient for recovering plastics and non-ferrous metals and the sequencing of these processes is one factor in developing a cost-effective recovery process.
Some materials absorb electromagnetic energy, such as microwave or radio wave energy, in a process called dielectric heating. Some molecules are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other. The most common dipole molecule is water. When exposed to microwaves or radio waves these dipoles rotate as they try to align themselves with the alternating electric field induced by the microwave or radio wave beam. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. For example, materials that tend to heat when exposed to microwaves include wood, rubber and foam. In contrast, other materials such as plastics are not heated when exposed to microwave radiation.
Fluorescent dyes have been used as tracers, such as to detect liquid leaks or identify the location of an object (the military uses fluorescent dyes to mark the location of a downed airplane in a body of water). When exposed to ultraviolet (UV) light or light of other wavelengths, these dyes fluoresce, indicating the presence of the dye. As such, porous materials could absorb dye-bearing liquid and UV light could be used to detect the presence of this liquid in the pores of the material. Wood, rubber, and foam would be examples of porous materials, while plastics and metals would typically not be porous.
In view of the foregoing, a need exists for cost-effective, efficient methods and systems for sorting materials, such as materials seen in a recycling process, including plastics and metals, in a manner that facilitates revenue recovery while also reducing landfill. Such methods and systems may employ electromagnetic radiation or fluorescent dyes to distinguish the plastics and metals from other materials, such as wood, rubber, and foam.